Line filters with so-called “X” capacitors connected between supply lines of a power supply network, for example, between a phase conductor and a neutral conductor or between two phase conductors are widely known. They are used to suppress differential-mode interference voltages, i.e., interference voltages between supply lines of the type caused by electrical loads.
FIG. 3 shows an example of a known line filter 1. Two X capacitors CX1 and CX2 and a discharge resistor RX are connected between a phase conductor Ph and a neutral conductor N. The line filter 1 further comprises two inductors L1 and L2 in the phase line Ph and the neutral conductor N, respectively.
The inductors L1 and L2 attenuate common-mode parasitic interference currents, i.e., interference currents that appear in identical phase on the phase conductor Ph and the neutral conductor N relative to the ground potential. Additional capacitors, known as “Y” capacitors, can additionally be arranged between the phase conductor Ph and the neutral conductor N, respectively, and a ground conductor. However, they are not shown in FIG. 3.
The X capacitor CX1 is arranged on the network-side upstream of the two inductors L1 and L2 of the phase conductor Ph and the neutral conductor N, respectively. The X capacitor CX2 is arranged downstream of the inductors L1 and L2. The X capacitors CX1 and CX2 suppress differential mode interference caused by a load at the output of the filter on the right side to prevent influencing of the network voltage as much as possible.
In particular, for computer switching mode power supplies, which have a relatively high power of generally more than 100 W, the X capacitors have relatively large dimensions. If the line filter 1 is disconnected from the power supply network, a relatively large charge remains in the X capacitors with a voltage that corresponds to the network voltage at the time of the interruption. This residual charge can cause dangerous electric shocks in the case of contact with a network plug, for example.
To prevent this, the discharge resistor RX connects between the phase conductor Ph and the neutral conductor N, and discharges the X capacitors CX1 and CX2 within a defined short time span when the network connection is interrupted. According to relevant specifications such as the VDE Standard “Information technology equipment—Safety, DIN EN 60950-1,” a discharge to less than 37% of the initial value must take place within a time of less than one second. Alternatively, the total capacitance of all X capacitors must be limited to a value of at most 100 nF.
One problem with the above-described circuit is that the discharge resistor RX inserted between the phase conductor Ph and the neutral conductor N leads to a power loss, both during operation of an electrical device downstream of the line filter and in a so-called “standby” state, in which the electrical device is supplied only with an auxiliary voltage. If the line filter for a switching power supply for a computer system is dimensioned at approximately 200 W, a power loss of roughly 100 mW results from a discharge resistor RX of 500 kΩ for example. If the computer system is in a standby state with a power consumption of 5 W, for example, the power loss amounts to 2% of the total power.
It could therefore be helpful to reduce power loss of a line filter, both when switched on and in the so-called “standby” operation of an electronic device coupled to the line filter. Electrical safety, particularly when pulling a power cord of the electronic device, should be preserved as well.